Previously, gravitational segregation and refined separation by filtration adsorption were used for treating oily wastewater from vessels. However, previous treatment equipment did not have the capability to treat emulsified oil contained in oily wastewater and failed to meet the requirements of current shipbuilding specifications. The existing technology was mainly a method of separation by chemical de-emulsification. This prior method required a large area for treatment equipment and produced large amounts of solid sediment that required expensive secondary treatment.
In order to meet the requirements of oily wastewater treatment for vessels, countries all over the world have invested vast human and material resources in research and development. Up to now, only the membrane separation method and the microbial and physical combinative method can actually be used for oily wastewater treatment for vessels. Chiefly employing the principle that water molecules, which are smaller than oil molecules, can pass through the micropores of membranes while the larger oil molecules are prevented from doing so, the membrane separation method separates oil and water in emulsified oil wastewater by means of ultra-filtration under a defined pressure. This method uses numerous kinds of membranes, including inorganic membranes, organic membranes, and inorganic/organic composite membranes. The pressure applied to the membranes during ultra-filtration is about 0.2˜0.4 MPa. The disadvantages of the membrane separation method are that the micropores easily become blocked or clogged and the actual lifespan of the membranes is short. Some of the oil molecules also adsorb to the wall of the membranes
In the microbial and physical combinative method, after the membrane separation method is performed, microorganisms specifically engineered for consuming hydrocarbons (such as oil) are cultivated and introduced into a biological reactor, together with the previously membrane-separated oily wastewater. With an input oil concentration of 70˜90 ppm in the wastewater, output concentration after treatment by the segregator is 15 ppm, which meets the discharge standards. The segregator has a treatment capacity of up to 86,000 gallons per month. Costs of maintenance and replacement are high for membranes, and the costs for the microbial and physical combinative method are also very high. Because of this, it is extremely difficult to promote and generalize the use of these processes in the global shipping industry.
The use of a metal anode electrolytic method has also been proposed for emulsified oil wastewater treatment. This method mainly involves electrolysis of dissoluble metal anode (iron anode) to produce metallic ferrous ions, employing the principle of electric charge equilibrium. Concurrently, metal ions form flocculent hydroxide in water that function as adsorbents, and the resulting oily floccules are floated to the top and removed. The method requires a treatment tank of large capacity and involves treatment of a huge amount of solid floccules, and the dissoluble electrodes used in this prior method have short lifespan. Therefore, the method is still not suitable for treatment of oily wastewater of vessels.